Traditionally lithographic printing systems have been used for high volume printing applications. In a typical lithographic printing system a continuous web of paper is supplied from a large reel and the paper is fed through successive print stations. Each print station has an impression cylinder that is outfitted with one or more patterned printing plates and applies one type of ink to the receiver according to the pattern on the printing plates.
More recently, high-volume inkjet printing systems have been introduced that form patterns of ink on a paper without using printing plates. Instead, in high-volume inkjet printing systems, digitally controlled inkjet printheads direct fine drops of ink across an air gap and onto a paper. High speed inkjet printing systems such as the Kodak Prosper Press Solutions such as the Prosper 1000 and 5000 printing systems, the Kodak Versamark V-Series Printing Systems including the VL Series of printing systems, the VX5000 printing systems and VT5000 printing systems all sold by Eastman Kodak Company, Rochester, N.Y., USA, have demonstrated the ability to provide high quality prints at high rates of production using inkjet printing. While high-speed inkjet printing systems such as those described above offer greater flexibility and adaptability than lithographic printing systems many popular types of paper used in lithographic printing today are not compatible.
In general, both inkjet inks and lithographic inks include a colorant material and liquid components. Conventionally, colorant comprises small particles of material such as pigments or dyes. The liquid components provide a medium in which the colorant can flow from a source onto the receiver. The liquid components of the ink can also perform additional functions as is known in the art. The liquid components are typically hydrophilic in nature such as water and certain alcohols and they are typically absorbed by the receiver or evaporate leaving the colorant on the receiver or in the receiver.
However, there are significant differences between inkjet inks and lithographic inks. One example of a difference between inkjet inks and lithographic inks can be found in the viscosity of these inks. Inkjet inks must flow easily so that they can be jetted from an inkjet nozzle. Accordingly, the viscosity of inkjet inks is typically relatively low.
The requirement that inkjet inks have lower viscosities creates another difference between inkjet inks and lithographic inks. Specifically, the lower viscosity of inkjet inks limits a colorant load that drop of inkjet ink can deliver. In contrast, lithographic inks are not jetted and equal size drop of lithographic ink can deliver a substantially greater colorant load. In some cases, a drop of inkjet ink can deliver a colorant load that is as little as one third to one fifteenth of the amount that an equal sized liquid drop of lithographic ink. In practice, several drops of inkjet ink are applied in order to transfer the same colorant load that a single drop of lithographic ink can transfer.
Still another difference between inkjet inks and lithographic inks can be found in the extent to which these inks spread when applied to a receiver. In particular, the lower viscosity and higher liquid volumes used to form a dot on a receiver using inkjet inks causes inkjet inks tend to spread across a receiver more than lithographic inks do. For example, for inkjet ink drops having drop volumes between approximately 2 and 10 pL generally have spherical-drop diameters of approximately 16 μm and 27 μm, respectively. Upon striking a non-absorbent receiver, the drops can spread by between 1.5 times the drop diameter and 3 times the drop diameter (e.g., as described in U.S. Pat. No. 6,702,425, which is incorporated herein by reference). This creates printed dot sizes of between 24 μm and 81 μm. In some systems, drops can spread by as much as 15 times the drop size (as described in U.S. Pat. No. 7,232,214, which is incorporated herein by reference), resulting in spot sizes between 30 μm and 150 μm. The large size of the ink dot after spreading limits the resolution that can be achieved using inkjet inks and can produce image artifacts such as granularity and mottle. (Small-drop-spread systems can also produce low-quality images because of the relatively lower proportion of the paper that is covered, e.g., as described in U.S. Pat. No. 5,847,721, which is incorporated herein by reference.).
Still another example of a difference between a inkjet ink and lithographic inks can be found in the size of the particulate type colorants that can be delivered by each. Here, the size of particulate components in inkjet ink is typically a fraction of the size of the particulate components in a lithographic ink. This is because the particulate in an inkjet ink must be small enough to be jetted through a small nozzle without clogging the nozzle. As lithographic inks are not jetted through a nozzle, lithographic inks can use particulate components that are much larger than those in inkjet inks.
The viscosity, size of the particulate components of an ink and the hydrophilic nature of receivers such as papers can have a significant impact on the extent to which an ink forms colors on a receiver. In particular, the lower viscosity and hydrophilic nature of inkjet inks can draw small sized particulate colorants in the inks into the fibers or within other surface variations of a receiver. This can cause at least a portion of the colorant to come to rest behind or within the fibers and lowers the effective density of a dot printed in this manner and can increase the amount of inkjet ink that must be transferred onto a receiver to form a dot with a particular color. However, in lithographic inks the greater colorant load, tolerance for lower viscosity and ability to deliver larger particle size of certain colorants combine to limit these effects.
In sum, the printing of a dot of a given color using inkjet inks requires the transfer of substantially more liquid onto a receiver than the printing of the same dot using lithographic inks on the same receiver and the success of an inkjet printing system in printing dots of a particular size and particular color on a particular receiver therefore ultimately is more dependent on the ability of the receiver to absorb the amounts of liquid than is the case with lithographic inks.
However, different types of receivers often provide different capacities to absorb and process the liquid components of ink that are during printing and it is difficult to use inkjet inks to form images on certain types of popular lithographic papers. For example, clay-coated graphic arts papers commonly used in lithographic printing (e.g., Potlatch Vintage Gloss, Potlatch Vintage Velvet, Warren Offset Enamel, and Kromekote papers), and non-absorbent papers such as polymer-coated papers used for photographic printing have limited ability to absorb liquid components of an ink. While such graphic arts papers may be capable of absorbing enough of the liquid components of a lithographic print to provide prints having a good appearance, such graphics arts papers do not have the ability to absorb the amounts of liquid that are transferred in inkjet printing. This allows inkjet inks to remain on the surface of such papers in liquid form for a period of time that allows the ink to overspread, to be smeared and transfer onto other parts of the printer or onto other receivers.
To address this problem special coatings have been applied to clay-coated papers. These coatings are designed to rapidly absorb and coalesce ink drops. Such coatings however can alter the feel, gloss, color or texture of the receiver which can render a print made with such coatings unacceptable to a customer who expects the print having to look and feel like lithographic print. Further, the special coatings increase the cost of the receivers and can influence the proper operation of the printer.
What is needed in the art therefore are methods and systems that that allows inkjet printing on a larger set of receivers including those that have less ability to absorb liquids while preserving the appearance and integrity of such receivers.